When using a laser beam to perform ocular surgery, the precise positioning of the laser beam's focal spot at a desired location in the tissue to be altered is important. Specifically, focal spot position accuracies within about ten microns (10 μm) or less are preferable. In addition to focal spot position, beam quality can affect the size and shape of the focal spot. In particular, laser beams with high beam quality are desirable because they can be used to produce relatively small, uniformly shaped focal spots.
For these treatment procedures, it is often desirable to use a patient interface to properly stabilize and align the eye with the surgical laser unit. In many instances, such stabilization may be best accomplished by placing the eye in direct contact with a patient interface of the laser unit. Typically, this involves placing the exterior surface of the cornea in contact with a posterior surface of a contact element (i.e. plate or lens) that is transparent to the surgical laser beam. In some cases, the posterior surface of the contact element may be flat, and, when applied to the cornea may flatten the cornea. Alternatively, the posterior surface of the contact element may be curved and substantially conforming to the exterior surface of the cornea. For this case, when the contact element is applied to the cornea, little or no flattening or distortion of the cornea occurs. In still other cases, a slightly curved contact element may be employed, slightly deforming, but not fully flattening the cornea when applied. Unlike a flat contact element, one advantage of using the curved/conforming contact element is that it does not cause distortions or wrinkles on the anterior surface of the cornea. These distortions or wrinkles can adversely affect a beam passing through the cornea and affect beam quality and focusability for beams used to treat target tissue that is located deeper in the eye than the cornea (e.g. the lens, retina, etc.).
In addition to the contact element, the patient interface typically includes a stabilizing device, such as a suction ring, that can be positioned directly against the eye (e.g. against the cornea) to hold the contact element against the cornea. An interface device can then be engaged with the stabilizing device and is used to fixedly hold the stabilizing device (e.g. the suction ring) relative to the laser unit. For example, U.S. Pat. No. 7,955,324 which issued to Melcher et al. for an invention entitled “Cornea Contact System,” and which is assigned to the same assignee as the present invention, discloses a system for this purpose.
Regardless of the type of contact element used (i.e. flat, slightly curved or curved and conforming to the cornea), the surfaces of the contact element have, heretofore, generally been manufactured to high dimensional tolerances. In this regard, variations in surface figure (i.e. shape) from the desired shape, as well as poor surface roughness, can affect a beam passing through the contact element. In the case of a curved contact surface which is generated using a turning technique, it can be relatively expensive to consistently obtain a contact surface that is free of surface irregularities that can affect beam position or beam quality.
Because of the high cost of manufacturing high-precision contact elements they are typically re-used on a number of different patients. Each use, in turn, requires the contact element to be sterilized and re-inspected for defects. This adds undesirable cost and complexity to the treatment procedure. If manufacturing defects could be compensated, lower cost methods of manufacturing contact elements, which do not necessarily involve turning, such as injection molding, may be used. In fact, these methods may lower the cost of manufacturing the contact element sufficient to allow for single use (i.e. disposable) contact elements, eliminating the need (and cost) for sterilization and inspection.
In light of the above, it is an object of the present invention to provide a method and apparatus for directing a surgical laser beam to a treatment area while compensating for the effects of irregularities (e.g. manufacturing defects) on the posterior surface of a patient interface. Another object of the present invention is to provide a method and apparatus which allow a low cost, dimensionally tolerant patient interface to be used in a laser procedure without reducing focal spot positioning accuracy or focusability. Yet another object of the present invention is to provide a method and apparatus which allows for alternative manufacturing methods to be used to produce a patient interface, such as injection molding, without reducing focal spot positioning accuracy or focusability. Still another object of the present invention is to provide a dimensional compensator for use with a patient interface that is easy to implement, is relatively simple to use, and is comparatively cost effective.